Methods in principle for conversion of light to electrical current (and vice versa) by means of organic electronics have been known for a few decades. A method that has made the technical breakthrough, and currently nearing readiness for the mass market, is that of multilayer structures as shown schematically, for example, in FIG. 1 for an organic light-emitting diode (OLED) or in FIG. 2 for an organic solar cell. Even though the efficiency of these components, specifically in the last few years, has experienced a distinct performance boost through the use of optimized classes of organic compounds, promising approaches are still giving rise to even higher quantum yields and hence even higher efficiencies coupled with lower material costs.
One of these approaches lies in the use of phosphorescent emitters, called triplet emitters, which find use, for example, in PHOLEDs (phosphorescent organic light-emitting devices). In view of the applicable quantum statistics, these are at least theoretically capable of achieving an internal quantum efficiency of 100%. This is in contrast with diodes having purely fluorescent emitters which, on the basis of the quantum statistics for the injected charge carriers, have a maximum internal quantum yield of only 25%.
Considering the internal quantum yield alone, organic electronic components which utilize a phosphorescence-based conversion of power to light (and vice versa) (triplet emitters/emission) are if anything suitable for providing a high luminescence (cd/m2) or efficiency (cd/A, lm/W). In the field of compounds capable of triplet emission, however, several boundary conditions have to be noted. Phosphorescence also occurs in compounds of the elements of the fourth and fifth periods of the Periodic Table; however, the complexes of the metals of the 6th period have become established in the abovementioned applications. According to the position of the elements in this period, the origin of phosphorescence is weighted differently within the orbital structure of the complexes.
In the case of the lanthanoids, both the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) are predominantly metal-centered, meaning that the contribution of the ligand orbitals is relatively small. The effect of this is that the emission wavelength (color) of the emitters is determined almost exclusively by the band structure of the metal (examples: europium=red, terbium=green). Because of the significant shielding of the f electrons of these metals, ligands coupled to the metal are capable of splitting the energies of the fn configuration of the metals only by about 100 cm−1, such that the spectroscopy differs considerably from that of the f ions by virtue of their ligand field of the d ions. In the case of ions of the lanthanides, the color results from transitions from f orbitals to unoccupied s, p and d orbitals.
Going along the period to the elements osmium, iridium, platinum and gold, ligand fields split the metal orbitals by a factor 10-100 times more than in the case of lanthanoids. It is thus possible, through variation of the ligands, to produce virtually the entire visible wavelength spectrum with these elements. The strong coupling of the orbital angular momentum of the metal atom to the spin angular momentum of the electrons results in phosphorescence in the emitters. The HOMO here is usually metal-centered, whereas the LUMO is usually ligand-centered. The radiative transitions are therefore referred to as metal-ligand charge transfer transitions (MLCT).
Both OLEDs and OLEECs (organic light-emitting electrochemical cells) are currently using almost exclusively iridium complexes as emitters. In the case of the OLEDs, the emitter complexes are uncharged; in the case of the OLEECs, ionic, i.e. charged, emitter complexes are utilized. However, the use of iridium in these components has a serious drawback. Annual production of iridium is well below 10 t (3 t in 2000). This means that the material costs make a significant contribution to the production costs of organic electrical components. An additional factor is that iridium emitters are incapable of efficiently forming the entire spectrum of visible light. For example, there is a comparative shortage of stable blue iridium emitters, which is a barrier to flexible use of these materials in OLED or OLEEC applications.
In the more recent literature, by contrast, there are some approaches which propose “triplet harvesting” with non-iridium-based emitters as well. For example, Omary et al. in “Enhancement of the Phosphorescence of Organic Luminophores upon interaction with a Mercury Trifunctional Lewis Acid” (Mohammad O. Omary, Refaie M. Kassab, Mason R. Haneline, O. Elbjeirami, and Francois P. Gabbai, Inorg. Chem. 2003, 42, 2176-2178) refer to the possibility of attaining adequate phosphorescence of purely organic emitters through the use of mercury. As a result of the heavy atom effect of mercury in a matrix composed of organic ligands, a singlet-triplet/triplet-singlet transition of the excited electrons of the organic matrix is enabled in quantum-mechanical terms (ISC, intersystem crossing), which results in a distinct reduction in the lifetime of the excited electronic (triplet) states and avoids unwanted saturation of the population of these states. The cause of this mechanism is the spin-orbit coupling of the heavy mercury atom to the excited electrons of the organic matrix. A disadvantage, by contrast, is that the use of mercury is problematic for reasons relating to toxicity and environmental policy.
One way of obtaining an adequate quantum yield based on purely organic phosphorescent emitters is described, for example, by Bolton et al. in NATURE CHEMISTRY 2011, 1-6 (“Activating efficient phosphorescence from purely organic materials by crystal design”, Onas Bolton, Kangwon Lee, Hyong-Jun Kim, Kevin Y. Lin and Jinsang Kim. NATURE CHEMISTRY, 2011, 1-6). The idea presented here is that the incorporation of heavy halides into a crystal composed of an organic matrix leads to high quantum yields by virtue of phosphorescent organic emitters. A disadvantage of this solution, by contrast, is that this effect seems to require a certain distance between the heavy halides and the organic matrix and a crystalline structure. This would be a barrier to inexpensive industrial manufacture of organic components.
WO 2012/016074 A1, by contrast, describes a thin layer comprising a compound of the formula
where Ar1 and Ar2 are each independently a C3-30 aromatic ring; R1 and R2 are a substituent; a and b are each independently an integer from 0 to 12, where, when a is 2 or more, each R1 radical may be different than the others, and two R1 radicals may be joined to one another in order to form a ring structure, and, when b is 2 or more, each R2 radical may be different from the others and two R2 radicals may be joined to one another, in order to form a ring structure; A1 is some kind of direct bond, —O—, —S—, —S(═O)—, —S(═O)2—, —PR3—, —NR4— and —C(R5)2—; R3 is a hydrogen atom or a substituent; R4 is a hydrogen atom or a substituent; R5 is a hydrogen atom or a substituent and two R5 radicals may be different from one another; E1 is a monovalent radical having 50 or fewer carbon atoms; L1 is a ligand having 50 or fewer carbon atoms; c is an integer from 0 to 3, where, when c is 2 or more, each L1 radical may be different from the others; and any combination of a combination of E1 and Ar1 and a combination of E1 and Ar2 may form a bond; and, when c is 1 to 3, any combination of a combination of L1 and E1, a combination of L1 and Ar1, a combination of L1 and Ar2 and a combination of L1 and L1 may form a bond. A disadvantage, by contrast, is that the compounds described have only an inadequate quantum yield and do not have sufficient stability in solution, such that they break down.
DE 103 60 681 A1 discloses main group metal diketonato complexes of the following formula:
as phosphorescent emitter molecules in organic light-emitting diodes (OLEDs), in which M may be Tl(I), Pb(II) and Bi(III). Additionally disclosed are the use of these main group metal diketonato complexes as light-emitting layers in OLEDs, light-emitting layers comprising at least one main group metal diketonato complex, an OLED comprising this light-emitting layer, and devices containing an OLED of the invention. In experiments, by contrast, it was shown that the abovementioned compounds, synthesized with strict exclusion of water, do not exhibit any phosphorescence-based emission after electronic excitation. It is very likely that the phosphorescent emission cited originates from oxo clusters that cannot be determined specifically, which have formed in an uncontrolled manner, for example as a result of hydrolysis in the course of preparation. A disadvantage of this specific solution is that the Π system of these acetylacetonate ligands, especially of the fully fluorinated variants described, is not well-developed and, being the sole phosphorescence emitter, permits only low phosphorescence yields.
If stated at all in the documents cited above, the phosphorescence emitters are prepared by standard methods, for example deposition/reaction or crystallization from solution or coevaporation. No more specific methods for production of particularly effective layers including particularly advantageous phosphorescence emitter complexes are disclosed.